Evidence is accruing that serum antibodies to surface polysaccharides, including lipopolysaccharides of Gram-negative organisms, confer protective immunity. The Vi has been licensed by the World Health Organization and the FDA as a vaccine for typhoid fever. To overcome the age-related and T-independent properties of this capsular polysaccharide, the Vi was bound to medically-useful proteins by several synthetic schemes. A Phase 1 study confirmed the improved immunologic properties of Vi as a conjugate made with the B-subunit of the heat-labile toxin of Escherichia coli or a recombinant Pseudomonas aeruginosa exoprotein A. The Vi was bound to the protein by a bifunctional hetero linker, SPDP. Attempts were made to bind the Vi to a protein with adipic acid dihydrazide. A semisynthetic Vi was prepared by derivatizing pectin, a plant polysaccharide whose repeat unit is alpha-D-(l ->4)GalA, with acetic anhydride. The product was identical to the Vi with the exception that the C-2 of the treated pectin is O- rather -than N- acetylated. The immunologic properties of the di-O-acetylated pectin have been compared to the Vi. The structure of the Vi, especially the interaction of the carboxyl and the O-acetyl on C-2, was investigated by potentiometric titration, circular dichroism and reaction with a bulky organic reagent. The data fitted the conclusions drawn from construction of a space filling model in which the surface of the Vi was occupied by the acetyls which covered the carboxyls. Clinical studies confirmed the immunogenicity of a conjugate composed of the detoxified LPS of Vibrio cholerae. The serum antibodies are being analyzed the immunoglobulin composition of their vibriocidal activity. A new sero type cholera 0139 causing epidemics in India is under investigation for its LPS structure and possible cross-reactivity with other cholera. Another LPS, that of E. coli 0157, has been purified and bound to the exotoxin A of Clostridium welchii C (pig bel toxin). The resultant conjugate was immunogenic in mice and a lot suitable for clinical study is under synthesis with the objective of evaluating its safety, immunogenicity and ultimately, effective in preventing enteritis caused by this pathogens with especial reference to the complication of the hemolytic uremic syndrome.